Approximately 80,000 prosthetic heart valves are implanted each year. Such valves function by means of an occluding disc moving within a cage opening and closing a passage at a frequency of approximately 1,500 Hz. The cage is commonly formed of metal and hence the movement of the occluding disc produces a distinctive click sound which often is audible through the chest wall.
In some implantations either the cage, disc or both may deteriorate which can lead to the eventual failure of the valve. This may result from a separation of the cage struts or the disc becoming misshaped.
Since a properly operating valve produces a definite and distinctive sound pattern the condition and operation of the heart valve are subject to being acoustically monitored. Although this appears relatively easy to accomplish, in fact, it is most difficulty to do for several reasons. As noted above, prosthetic heart valves operate at frequencies on the order of approximately 1,500 Hz. Accordingly, conventional stethoscopes cannot be used to monitor the valve operation since their operating range is limited to frequencies below approximately 150 Hz as a result of the rubber tubes which connect the transducers to the ear piece. The tubes serve to dim out sounds at frequencies higher than approximately 150 Hz. A conventional microphone-type pickup placed against a patient's chest might pick up the valve sound but it would also pick up extraneous sounds such as the sound of blood rushing through the patient's circulatory system, breathing and in addition to lung noises, etc. the sounds associated with the prosthetic valve operation and thus the fine differences in the valve operation sought to be detected would be lost without extensive and elaborate filtering.
In view of the above, it is the principal object of the present invention to provide an improved stethoscope specifically adapted to pick up the sound of a prosthetic heart valve while automatically filtering out all other sounds;
a further object is to provide such a device which may be packaged in a housing convenient for application to a patient's chest;
a still further object is to provide such a device which may readily be tuned to a particularly prosthetic valve so that any change in the valve condition will quickly be detected;
still further objects will be apparent from the following description of the invention.